1. Field of the Invention
The present invention relates to a secondary cell with an electrode jelly roll in which a cathode, separator, and anode are wound together, and more particularly, a secondary cell with an electrode jelly-roll, which is improved in structural and thermal stabilities.
2. Description of the Related Art
Secondary cells are rechargeable and can be made into a smaller size with high capacity. Typical examples of secondary cells include nickel-metal hydride (Ni-MH) cells, lithium cells, lithium ion cells (Li-ion), and polymer lithium cells. Secondary cells are classified into cylindrical cells, rectangular cells, pouch type cells containing electrode jelly-rolls in pouches.
A secondary cell includes an electrode jelly-roll wound to be circular or elliptical. The electrode jelly-roll is attained by coating a substrate with an active material, and drying, roll-pressing and cutting the substrate to form a cathode and anode, and by winding the cathode and anode with a separator therebetween. A circular cell is formed by winding the electrode jelly-roll to have a circular cross-section, accommodating it in a cylindrical can, filling the can with an electrolyte solution, and sealing the can. A rectangular cell is formed by flattening the electrode jelly-roll under pressure and accommodating it in a rectangular can.
For example, a cylindrical secondary cell having a structure of an electrode jelly-roll, includes an electrode such as a cathode and anode can be manufactured by different methods according to the type of electrode. In general a cathode and anode are formed by coating a slurry containing a cathode active material and a slurry containing an anode active material on both sides of respective substrates, and drying, roll-pressing and cutting the substrates to a predetermined size. A separator is interposed between the cathode and the anode to prevent the cathode and anode from being electrically connected, and then wound in a roll.
The resulting electrode jelly-roll is placed in a can, a cap assembly is mounted on the top of the can to be connected with the cathode of the electrode jelly-roll, and the can is filled with an electrolyte solution and sealed, resulting in a cylindrical cell.
In such a cylindrical cell, the anode substrate contacts the inner wall of the can at the outer-side of the electrode jelly-roll, or an anode tap welded to the anode substrate contacts the bottom of the can. A tap at the core of the electrode jelly-roll, extending from the cathode substrate, is connected to the cap assembly. On the top and bottom surfaces of the electrode jelly-roll, insulating plates are placed to prevent short-circuiting between the cap assembly and the can.
Many efforts have been made to manufacture a high-capacity cell by tightly winding thin, long electrodes such that a large amount of active material can be incorporated into the cylindrical cell having the structure described above.
Another consideration to be taken into account in manufacturing a high-capacity cell is the mechanical and thermal stabilities of the cell.
A problem of thermal stability in a cell is caused as a result of the heat generated from the reaction in the cell cannot be effectively dissipated. This problem occurs when the electrodes of the cell are tightly wound so that the heat generated from the inside cannot be effectively dissipated to the outside. As a result, the temperature of the cell continues to rise and a thermal runaway phenomenon occurs, thereby degrading the stability of the cell.
A problem of mechanical stability in a cell refers to a reduction in cell stability when the electrodes are damaged due to external environments such as physical impacts.
An effort to improve the heat-dissipating structure of a cell has been made to prevent a reduction in thermal stability of the cell. In particular, U.S. Pat. No. 5,571,632 for Nonaqueous Electrolyte Solution Secondary Cell and Method for Producing the Same by Teramoto discloses a “non-aqueous electrolyte secondary cell” having a structure in which the cathode is welded to an inner Al tube and the anode is welded to a Ni foil on the outer side of the electrode jelly-roll.
To manufacture the secondary cell having the structure described above, an inner tube is formed through the can, the electrode is welded to the inner tube, and a separate member is welded to the outer side of the electrode jelly-roll. This structure of the secondary cell adds complexity to the manufacturing process and therefore is burdensome. Also, the structure of the secondary cell differs from that of a conventional cylindrical cell, so equipment commonly used to manufacture the secondary cell should be replaced. In addition, incorporation of such a large-volume inner tube limits to wind the electrode tightly for a high-capacity, compact cell.